Detecting the stoichiometric end point of phosgenation reactions

ABSTRACT

A method is provided for determining the stoichiometric end point of phosgenation reactions which produce polycarbonates and chloroformates, respectively, by monitoring the rate of heat generated by the reaction mixture per unit of phosgene utilized.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser.No. 855,975, filed Apr. 25, 1986, now U.S. Pat. No. 4,722,995, assignedto the same assignee as the present invention and incorporated herein byreference.

BACKGROUND OF THE INVENTION

This invention relates to a method for determining the stoichiometricend point of phosgenation reactions for producing polycarbonates andchloroformates. More particularly, it relates to methods for detectingthe increase in the rate of heat generated at about the stoichiometricend point of reactions between phosgene and organo-hydroxy compounds.

Both polycarbonates and chloroformates are known classes of organiccompounds. Polycarbonate polymers are known for their good engineeringproperties and inherent flame resistance. They can be obtained byreaction of aromatic dihydroxy compounds, such as bisphenol A, withphosgene. A polycarbonate for the purposes of this invention is anypolymer having carbonate groups prepared from the use of phosgene. Anaromatic polycarbonate has at least some of the carbonate groupsattached to an aromatic nucleus. The chloroformates, in particular,oligomeric carbonate chloroformate mixtures, are prepared from phosgeneand organo-hydroxy compounds. Mixtures of bischloroformate oligomers offormula I and monochloroformate oligomers of formula II are made fromdihydroxy compounds. ##STR1## wherein R is a divalent aliphatic,alicyclic or aromatic radical and n is at least 1 and the number averagefor n is preferably less than 3. The reaction is similar topolycarbonate synthesis; however, a polycondensation catalyst is notused. Chloroformates have been shown to be useful as intermediates inthe preparation of cyclic carbonate oligomers which may be converted tovery high molecular weight polycarbonates as disclosed in copendingapplication Ser. No. 704,122, filed Feb. 22, 1985 assigned to the sameassignee as the present invention.

There are a number of known methods for preparing choloroformates andpolycarbonates by reaction with phosgene. Each reaction is normallyconducted interfacially; that is, in a mixed aqueous-organic systemwhich results in the recovery of the product in the organic phase. Fordetailed descriptions of phosgenation reactions which providepolycarbonates, reference is made to the following U.S. Pat. Nos.:3,155,683, 3,274,214, 3,386,954, 3,422,119, 4,129,574, 4,216,305,4,197,394, 4,360,659, 4,224,434 and to the procedures described inEncyclopedia of Polymer Science and Technology, Polycarbonates (1969),Vol. 10, pp. 710-764, Interscience Publishing. For a detaileddescription of chloroformate synthesis, reference is made to U.S. Pat.Nos. 3,312,661, 3,959,335, 3,974,126 and 3,966,785, which preparebischloroformate compositions by reacting a water soluble salt of analkylidene diphenol with phosgene in an aqueous system employing anorganic diluent.

Although the preparation of aromatic polycarbonates and chloroformateswith phosgene is well known, until recently a substantial excess of thecalculated stoichiometric amount of phosgene was added to the reactionvessel to insure that all the initially added organo-hydroxy compoundstarted with would react.

U.S. Pat. No. 4,378,454, issued Mar. 29, 1983, discloses a method fordetermining the end point of the polycarbonate polymerization reaction.This method is based on the known solubility of phosgene in the organicsolvent utilized. The solubilized phosgene reacts immediately with thebisphenol and, since there is insufficient bisphenol, once thestoichiometric end point is reached the additional solubilized phosgenecan be detected by a phosgene color test, described more particularly inU.S. Pat. No. 4,378,454.

In U.S. Pat. No. 4,506,067, a method is described wherein thestoichiometric end point of the preparation of aromatic polycarbonateresin with phosgene is determined by the increase in phosgene gasoccurring in the vapor phase of the reactor.

It has now been discovered that at or slightly after the time the endpoint of the aromatic polycarbonate or chloroformate preparation hasbeen reached, there is a substantial increase in the rate of heatgenerated by the reaction mixture per unit of phosgene utilized. Thisincrease in heat generation can be detected by any standard means andwill signal the end point of the desired reaction. Phosgene addition canthen be terminated, thereby saving the extra phosgene which would havebeen added to insure the achievement of the reaction end point. Reducingthe amount of phosgene also reduces the amount of time for eachreaction, whether batch or continuous, thus increasing the effectivecapacity of present plant equipment.

DESCRIPTION OF THE INVENTION

In accordance with the invention there is a method for detecting thestoichiometric end point of a reaction between phosgene and anorgano-hydroxy compound in a reaction mixture which contains an aqueoussolution of alkali metal hydroxide or an alkaline earth hydroxide and anorganic solvent solution in which phosgene is normally soluble. Thismethod comprises detecting the increase in the rate of heat generated bythe reaction mixture at about the stoichiometric end point of reaction.

Although this invention can be used for reactions between phosgene and awide variety of organo-hydroxy compounds, it is particularly useful inprocesses which produce aromatic polycarbonates and mixtures ofoligomeric carbonate chloroformates. These well known reactions can becarried out under standard conditions normally applicable, such asatmospheric pressure and a temperature of about 10° to 40° C.

Not being bound by theory, it is believed the increase in the rate ofheat generated by the reaction mixture is due to the increase inhydrolysis of phosgene at about the stoichiometric end point of thereaction between phosgene and organo-hydroxy compounds. Hydrolysis ofphosgene takes place in the presence of alkali metal hydroxide or analkaline earth hydroxide, as illustrated by equations 1 and 2,respectively

    COCl.sub.2 +4XOH→X.sub.2 CO.sub.3 +2XCl+2H.sub.2 O, (1)

    COCl.sub.2 +2Y(OH).sub.2 →YCO.sub.3 +YCl.sub.2 +2H.sub.2 O, (2)

wherein X is an alkali metal such as sodium, potassium, lithium andcesium and Y is an alkaline earth metal such as calcium. The heat ofreaction for the hydrolysis of phosgene by excess aqueous NaOH insolution is about -104 kcal/mole COCl₂, which is higher than the heatgenerated from the reactions of phosgene and organo-hydroxy compounds.In the production of phenyl-chloroformate from phenol and phosgene, theheat of reaction is substanially less than the heat generated by thehydrolysis reaction.

In synthesizing polycarbonate from a medium containing water usingphosgene gas, the formation of the carbonate linkage may be expected toproduce a quantity of heat approximately equal to that generated by thesynthesis of diphenylcarbonate via equation 3. ##STR2## which is alsosubstantially less than the heat generated by the correspondinghydrolysis reaction.

To provide a rapid increase in the rate of heat generated by thereaction mixture, a hydrolysis medium must be present, i.e., an aqueoussolution of an alkali metal hydroxide (or alkaline earth metalhydroxide) must form part of the reaction mixture. The preferred alkalimetal hydroxide is NaOH. The preferred alkaline earth metal hydroxide isCa(OH)₂. To avoid complete hydrolysis of the phosgene introduced to thereaction mixture, an organic solvent solution in which phosgene issoluble must be present. Organic solvents which can be utilized includethe halogenated organics such as methylene chloride, chloroform, carbontetrachloride, trichloroethylene, ethylene dichloride, chlorobenzene andthe like, hydrocarbon solvents including benzene, toluene and the likeas well as heteroatom containing solvents such as pyridine, lutidine,dimethyl sulfoxide, tetrahydrofuran, dioxane, dimethylformamide,nitrobenzene and the like. Methylene chloride is the preferred solvent.

These two liquid components are generally immiscible and the reactionmixture is two phases, an aqueous phase that contains the alkali metalhydroxide and some organo-hydroxy compound and an organic phase whichcontains phosgene and the product formed. Additional organo-hydroxycompound may be present as a solid. The reaction takes place at theinterface of these two liquid phases. The type and quantity of organicsolvent employed usually solubilizes all the product; however,significant quantities of the product may be insoluble in the solvent.Extra quantities of organic solvent can be used but in general areunnecessary and merely add to the cost of carrying out the process.

For polycarbonate synthesis, a polycondensation catalyst is used andtypically is present in the organic phase. This polycondensationcatalyst can be any hydrogen halide acceptor commonly employed ininterfacial polycondensation reactions and is preferably a tertiaryamine. Illustrative of well-known catalysts are the following;trimethylamine, triethylamine, allyldiethylamine, benzyl dimethylamines,dimethylphenethylamine, N-methylpiperidine and the like. In addition totertiary amines which are preferred, there also can be used as thepolycondensation catalyst, quaternary salts such as quaternary ammoniumsalts and quaternary phosphonium salts.

Any amount of polycondensation catalyst can be employed. However,generally, effective mole proportions relative to the organo-hydroxycompound are within the range of from about 0.25% to about 2% per mole.

In addition to the polycondensation catalyst, a chain-stopper such as amonohydric phenol can be used for polycarbonate synthesis for molecularweight control. There can be used from 0 to 10 mol percent based ontotal phenolic monomer in the mixture and preferably 0.5 to 6 mole %.

The organo-hydroxy compounds which can be utilized in this invention arehydroxy substituted organic compounds of formula III,

    R.sup.2 (OH).sub.x,                                        (III)

wherein R² is an aliphatic, alicyclic or aromatic radical and x is atleast 1. The sole reactive groups on these compounds must be thehydroxyl radicals which provide reactive terminal protons. Theorgano-hydroxy compounds defined by formula III are generally useful inchloroformate synthesis. Suitable R² values for chloroformate synthesiswhen x is 1 are 1-butyl, 2-butyl, 1-hexyl, cyclohexyl and phenyl. Wherex is 2 and R² is a divalent aliphatic or alicyclic radical, illustrativevalues for R² include ethylene, propylene, trimethylene, tetramethylene,hexamethylene, dodecamethylene, poly-1,4-(2-butenylene),1,3-cyclopentylene, 1,3-cyclohexylene and 1,4-cyclohexylene, includingsubstituted derivatives thereof. Illustrative inert substituents includealkyl, cycloalkyl, halo and nitro.

The organo-hydroxy compounds of formula III which are dihydric phenols,i.e., where x is 2 and R² is a divalent aromatic radical, areparticularly suitable for both chloroformate and polycarbonatesynthesis. Some of these are represented by the formula ##STR3## whereinA is a divalent hydrocarbon radical containing 1-15 carbon atoms, or##STR4## X is independently hydrogen, chlorine, bromine, fluorine, or amonovalent hydrocrbon radical such as an alkyl group of 1-4 carbons, oran aryl group of 6-8 carbons such as phenyl, tolyl, xylyl and n is 0 to1.

One group of such dihydric phenols are those illustrated below:

1,1-bis(4-hydroxyphenyl)butane

1,1-bis(4-hydroxyphenyl)isobutane

1,1-bis(4-hydroxylphenyl)-1-phenyl ethane

1,1-bis(4-hydroxyphenyl)-1,1-diphenyl methane

1,1-bis(4-hydroxyphenyl)cyclooctane

1,1-bis(4-hydroxyphenyl)cycloheptane

1,1-bis(4-hydroxyphenyl)cyclohexane

1,1-bis(4-hydroxyphenyl)cyclopentane

2,2-bis(3-propyl-4-hydroxyphenyl)decane

2,2-bis(3,5-dibromo-4-hydroxyphenyl)nonane

2,2-bis(3,5-isopropyl-4-hydroxyphenyl)nonane

2,2-bis(3-ethyl-4-hydroxyphenyl)octane

4,4-bis(4-hydroxyphenyl)heptane

4,4'-dihydroxy-diphenyl-2-chlorophenyl methane

4,4'-dihydroxy-diphenyl-2,4-dichlorophenyl methane

4,4'-dihydroxy-diphenyl-p-isopropylphenyl methane

4,4'-dihydroxy-diphenylnaphthyl methane

4,4'-dihydroxy-3-methyl-diphenyl-2,2-propane

4,4'-dihydroxy-3-cyclohexyl-diphenyl-2,2-propane

4,4'-dihydroxy-3-methoxy-diphenyl-2,2-propane

4,4'-dihydroxy-3-isopropyl-diphenyl-2,2-propane

4,4'-dihydroxy-3,3'-dimethyl-diphenyl-2,2-propane

3,3-bis(3-methyl-4-hyrodxyphenyl)hexane

3,3-bis(3,5-dibromo-4-hydroxyphenyl)hexane

2,2-bis(3,5-difluoro-4-hydroxyphenyl)butane

2,2-bis(4-hydroxyphenyl)propane(Bisphenol A)

1,1-bis(3-methyl-4-hydroxyphenyl)ethane

1,1-bis(4-hydroxyphenyl)methane

6,6'-dihydroxy-3,3,3',3'-tetra-1,1'-spirobiindane

1-(4-hydroxyphenyl)-1,3,3-trimethyl-6-indanol.

Another group of dihydric phenols useful in the practice of the presentinvention include the dihydroxyl diphenyl sulfoxides such as forexample:

bis(3,5-diisopropyl-4-hydroxyphenyl)sulfoxide

bis(3-methyl-5-ethyl-4-hydroxyphenyl)sulfoxide

bis(3,5-dibromo-4-hydroxyphenyl)sulfoxide

bis(3,5-dimethyl-4-hydroxyphenyl)sulfoxide

bis(3-methyl-4-hydroxyphenyl)sulfoxide

bis(4-hydroxyphenyl)sulfoxide.

Another group of dihydric phenols which may be used in the practice ofthe invention includes the dihydroxyaryl sulfones such as, for example:

bis(3,5-dilisopropyl-4-hydroxyphenyl)sulfone

bis(3,5-methyl-5-ethyl-4-hydroxyphenyl)sulfone

bis(3-chloro-4-hydroxyphenyl)sulfone

bis(3,5-dibromo-4-hydroxyphenyl)sulfone

bis(3,5-dimethyl-4-hydroxyphenyl)sulfone

bis(3-methyl-4-hydroxyphenyl)sulfone

bis(4-hydroxyphenyl)sulfone.

Another group of dihydric phenols useful in the practice of theinvention includes the dihydroxydiphenyls:

3,3',5,5'-tetrabromo-4,4'-dihydroxydiphenyl

3,3'-dichloro-4,4'-dihydroxydiphenyl

3,3'-diethyl-4,4'-dihydroxydiphenyl

3,3'-dimethyl-4,4'-dihydroxydiphenyl

p,p'-dihydroxydiphenyl.

Another group of dihydric phenols which may be used in the practice ofthe invention includes the dihydric phenol ethers:

bis(3-chloro-5-methyl-4-hydroxyphenyl)ether

bis(3,5-dibromo-4-hydroxyphenyl)ether

bis(3,5-dicholor-4-hydroxyphenyl)ether

bis(3-ethyl-4-hydroxyphenyl)ether

bis(3-methyl-4-hydroxyphenyl)ether.

It is, of course, possible to employ a mixture of two or more differentdihydric phenols or a mixture of a dihydric phenol and an aliphaticalcohol in the reactions of the invention. It is also possible to employtwo or more different dihydric phenols or a copolymer of a dihydricphenol with a glycol or with a hydroxy-terminated polyester.

The increase in the rate of heat released by the reaction mixture hasbeen found to be sufficient to provide an adequate signal for the endpoint of the reaction.

Any means for detecting an increase in heat generated per unit ofphosgene is suitable for use in this invention. These means may varywith the reaction conditions and the system utilized. If is preferableto maintain the delivery rate of phosgene to the reaction mixture at aconstant value to avoid an increase in heat generation by an increase inthe rate of reaction.

Where the reaction mixture is below the reflux temperature, the increasein the rate of heat generated per unit of phosgene can be determined bymonitoring the temperature of the reaction mixture. The increase in heatgeneration will be reflected by a temperature rise. This temperaturerise can be detected by conventional means such as a thermocouple.

Where the reaction mixture is at reflux, the temperature is constant andthe increase in heat generated per unit of phosgene can be determined bymonitoring the solvent vaporization rate. Any method for monitoring theincrease in vaporization is suitable. Where a condenser is utilized toreturn the vapors to the reaction mixture, the velocity of the vaporsthrough the condenser can be measured by monitoring the pressure.

Alternatively, the position of the vapor/condensate interface in thecondenser can be monitored. In the process of this invention, the gas iscarbon dioxide, produced by hydrolysis of chloroformates or of phosgene,and the vapor is the uncondensed solvent, e.g., methylene chloride.Since the vapor is denser than the gas, a stable interfacial region isformed within the condenser, the position of which is dependent on thetotal rate of vapor to be condensed. The greater the amount of vapor,the greater the total area of the condenser required to condense it.Thus, in the usual case of a vertical condenser, additional refluxingvapor from additional heat generated near endpoint reactions forces theinterface to advance up in the condenser tubes. A rise in height of thisinterface corresponds to an increase in vaporization rate. Thisphenomenon is described more particularly by Kosky and Jaster in theJournal of Electrostatics, Vol. 6(1979), pp. 107-119, ElsevierPublishing Co., The Netherlands. This interface can be monitoredvisually in glass equipment or by detecting the temperature differencebetween the gas filled portions and the vapor filled portions of thecondenser. The position of this interfacial region can be detected byappropriately situated thermocouples or by a 2-junction null detectingthermocouple strategically placed to monitor the passage of theinterfacial zone as it passes the upper junction.

Another method for detecting the increase in vaporization where acondenser is utilized is to monitor the increase in temperature of thecoolant for the condenser. This requires sensitive equipment since thetemperature increase may be slight where large volumes of coolant areused.

Where the reaction mixture is at reflux and contains two or morevaporizable components having different vaporization temperatures, thereaction mixture is not in a steady state where one or more of thesecomponents escapes the system. Under such conditions, there is a changein the composition of the reaction mixture and vapors as the reactionproceeds. This causes a change in the temperature for the reactionmixture and the temperature of the vapors and reaction mixture willincrease. This increase in temperature can be monitored to determine theend point of the reaction.

The additional vaporizable component within organic solvent solutionmust be inert. Examples of suitable inert vaporizable components areinert fluorocarbon refrigerants having a boiling point in the range ofabout -10° C. to 30° C. These include halogenated fluorocarbons such asCCl₃ F, CHCl₂ F, CClF₂ --CClF₂, CCl₂ F--CF₃, CH₂ Cl--CF₃, CH₃ --CClF₂,CBrClF₂, CF₂ I--CF₃, CHClF--CClF₂, and the like, prefluorinated carboncompounds of from 4 to 5 carbon atoms and fluorinated ethers of from 2to 4 carbon atoms; examples of which include n-perfluorobutane,perfluoroisobutane, cyclic-octafluorobutane, tetrafluorodimethyl ether,perfluorodiethyl ether, 1,1-difluorodimethyl ethers, etc. Mixtures ofsuch refrigerants are also suitable. Those fluorocarbon refrigerantswhich have a boiling point below 15° C., such as CClF₂ --CClF₂, arepreferred. Other inert vaporizable components and refrigerants whichsatisfy the boiling point and vapor pressure requirements describedabove are suitable and are considered within the scope of thisinvention.

In order that those skilled in the art will be better able to practicethe present invention, the following examples are given by way ofillustration to demonstrate the rapid rise in the heat generation rateper unit phosgene. There are numerous means by which this increase canbe detected and it is not intended to limit this invention to theembodiment described.

EXAMPLE 1

A 2-liter Morton flask was charged with bisphenol A (BPA) (114.14 g;0.50 mol), methylene chloride (400 ml.), and Freon 114 (CFCl₂ CFCl₂ ;100 ml.) and was fitted with a chilled spiral condenser (condensertemp=6° C.), mechanical stirrer, thermometer, pH meter, NaOH additionfunnel, and phosgene bubbler. The reaction was vented to a liquidnitrogen-cooled trap. Phosgene was bubbled through the rapidly stirredmixture at a rate of about 2.0 gm./min. Aqueous NaOH (5M) was addedconcurrently at such a rate as to maintain the pH in the range of 2-5.The reaction temperature increased from 8° C. at the start of reactionto 15° C. within 4 minutes after which a temperature of 15°-20° C. wasmaintained by keeping the dual solvent system at reflux. Some of therefrigerant was permitted to escape the system at the endpoint of thereaction, which allowed the rate of heat generation by the reactionmixture to be monitored with the thermometer. A summary of time andtemperature data is indicated in Table I.

                  TABLE I                                                         ______________________________________                                        Preparation of BPA-Bischloroformate                                           Time (min)                                                                            Temp (° C.)                                                                       pH    Δ/10 min.                                                                         Comments                                   ______________________________________                                        10.0    17         3.3   --        steady reflux                              15.5    17         3.0   --                                                   20      18         3.5   1° C.                                         27.5    19         4.5   --                                                   28.5    20         2.0   --                                                   29.5    20         2.0   2° C.                                         31      21         1.9   --                                                   35      21         3.0   --        BPA present,                                                                  steady reflux                              40      21         2.4   1° C.                                         45      21         4.1   --                                                   49.5    23         1.8   2° C.                                         51      24         4.8   --        v. slight BPA                                                                 present                                    52      25         3.5   --        BPA gone                                   55      28         3.5   10° C.                                                                           reaction                                                                      terminated                                 ______________________________________                                    

The temperature began to rise quickly after 50 minutes (ΔT/10 min.=10°C.), indicating the end point of reaction. When the temperature reached28° C., the flow of phosgene was terminated. The reaction was quenchedby sparging with nitrogen for 30 minutes (to remove any excessphosgene). The methylene chloride layer was washed with water and 1M HCland endcapped by the addition of an equimolar mixture of phenol andtriethylamine. A 2-ml. sample was removed for hplc analysis. Inspectionof the liquid nitrogen trap indicated that 15 ml. of solvent had escapedthe reactor.

Hplc analysis of the reaction product indicated 67%BPA-bischloroformate, 16% dimer bischloroformate, 5% trimerbischloroformate, and 2% tetramer bischloroformate. In addition, 4%bisphenol monochloroformate was formed.

As shown in Table I, the rapid rise in temperature signaled theconsumption of all BPA, i.e., the end point of the reaction.

Modifications of the above embodiment will be obvious to those skilledin the art and are considered to be within the scope of this invention.

EXAMPLE 2

Phosgene was bubbled at a rate of 2.57 gm/min through a reaction mixturein a 1 liter reactor similar to that of Example 1. The mixture consistedof bisphenol A (BPA) (114 gm; 0.50 mol), methylene chloride (550 ml),water (500 ml), endcapper (Phenol, 4.5 mol % based on BPA; 2.11 gm),polymerization catalyst (triethylamine, 2 mol % based on BPA; 1.01 gm),and 125 ml Freon 114 (CFCl₂ CFCl₂). A pH control system was used. Thereaction was vented to a dry ice/acetone trap. The pH was maintained at10.5 by addition of 50 wt % NaOH. The reactor temperature was initiallyat 16.3° C. and increased to 18° C. within 1 minute after the start ofphosgenation. The reaction temperature was maintained at 17.7°-18.0° C.during the next 17 minutes by refluxing the solvent mixture. After 19minutes of phosgenation, which is the stoichiometric endpoint of thereaction, the temperature rose rapidly, as shown in the table below:

                  TABLE II                                                        ______________________________________                                        Time (min) T(° C.)                                                                            pH     ΔT/10 min                                 ______________________________________                                         2         18.0        10.5   0                                               18         18.0        9.2    0                                               19         18.2        11.5   2                                               20         18.3        9.8    1                                               21         19.1        10.7   8                                               22         19.5        9.6    4                                               23         20.0        10.5   5                                               24         20.4        10.0   4                                               ______________________________________                                    

The product was sampled at 20 minutes and the GPC analysis showed aweight average MW of 18,000 and a polydispersity of 3.7. The sampletaken at 24 minutes was virtually identical by GPC, which indicates thatthe reaction was complete at the time of the rapid temperature rise(20-21 minutes).

Although the above examples are directed to only a few of the manyvariables which are in the scope of the present invention, it should beunderstood that the present invention is directed to a much broadervariety of ingredients and conditions as set forth in the descriptionpreceding these examples.

What is claimed is:
 1. A method for determining the polymerization endpoint between phosgene and organobishydroxy compound under interfacialpolycondensation reaction conditions comprising, monitoring the rate ofheat generated during the course of the polycondensation reaction untila rapid rise in heat is detected, per unit of phosgene added, at aboutthe stoichiometric end point of the reaction conducted in a mixturecomprising an aqueous medium which contains the organobishydroxycompound, an effective amount of a polycarbonate chain-stopper, apolycondensation catalyst, a member of the group consisting of alkalimetal hydroxides and alkaline earth metal hydroxides and an organicsolvent in which phosgene is soluble.
 2. A method in accordance withclaim 1, where the organobishydroxy compound is bisphenol A.
 3. A methodin accordance with claim 1, where the polycondensation catalyst istriethylamine.
 4. A method in accordance with claim 1, where thechain-stopper is phenol.